This invention relates to wavelength-division multiplexing in optical systems, and more particularly to a reflective apparatus for combining and/or separating wavelength channels.
An important function that must be provided in high quality optical networks is that of wavelength multiplexing and demultiplexing a plurality of signals of different wavelengths. In particular, an important device for performing this function is a channel adding/dropping filter of the type realized in [1-3] This device is be referred to herein as a wavelength interchanger, or simply interchanger. It is essentially characterized by a set of nonoverlapping wavelength passbands. In each passband the interchanger essentially behaves as an ordinary a 2xc3x972 switch. Thus the switch is characterized in each passband by two states, corresponding to the two permutations commonly referred to as cross and bar states. Moreover, the state in each passband can be chosen independently of those in the other passbands. Thus, the switch must include several controls, namely one for each passband. The interleaver must have low loss, preferably less than 3 dB, and it must be approximately characterized by rectangular transfer functions.
A technique that is often used for combining and separating signals of various channels in wavelength-division multiplexing is the wavelength router. A rectangular transfer function can be realized by concatenating two such routers, but such an arrangement is difficult to realize on a single wafer using conventional techniques for two reasons. The first reason is that the loss of a conventional router typically exceeds 3 dB, which would result in two concatenated routers having a total loss of more than 6 dB. Another reason is that the two-routers arrangement is difficult to realize on a planar wafer surface because of its size.
Thus there is a continuing need for a low loss interchanger having rectangular wavelength transfer functions.
In one embodiment of my new invention, a low loss wavelength interchanger is implemented using a waveguide grating router combined with (1) an input arrangement of N equally spaced waveguides that are connected to the router at equally spaced locations displaced by d on the input curve of the router and (2) an output arrangement of reflective elements causing each reflected signal to pass twice through the router. In this arrangement the waveguide grating router differs from a conventional router, in that suitable phase shifts xcfx86 are included in the grating elements, so as to cause each output image to split into N images of essentially identical intensities. Therefore, by applying to one of the N waveguides a particular wavelength, the router produces on the output image curve N interleaved sets of output images, wherein each significant order of the grating produces a particular image in each set, and N such images, respectively produced in the N sets by a particular order, have identical intensities. Accordingly, the output reflector arrangement includes N interleaved sets of reflective elements, such that each set reflects a particular set of images, and it applies to them the appropriate phase shifts xcex1, so as to cause the signal power to be only transferred to one particular waveguide. In one embodiment of this invention, each reflective element is realized using a multimoding imaging waveguide combined with a reflective termination. By using N=4 waveguides one can realize by this technique a 2xc3x972 arrangement in which two of the N=4 waveguides are used as input waveguides, and, the other two, are used as output waveguides. Each pair of adjacent reflective elements is adapted with suitable heaters to produce a variable phase shift between the reflection coefficients of the two elements. One thus realizes a 2xc3x972 arrangement in which the reflected input signals, after passing twice through the router, are recombined by the 2xc3x972 arrangement, thus producing a reflected signal from either input port to either output port of the 2xc3x972 arrangement, the output port being controlled by the phase shifts applied to the reflectors. One thus realizes a 2xc3x972 switch arrangement in which each of the input signals can be switched to either of the output ports under control of the phase shifts applied to the reflectors.
More specifically, my invention can be realized as a waveguide grating router comprising a grating having an input curve and an output image curve, the input curve having N, N greater than 2, equally spaced waveguides connected thereto, the output curve having reflective elements placed thereon, the waveguide grating router characterized by
the grating having a plurality of elements forming multiple paths through the router so as to transform a particular input wavelength applied to one of the N waveguides into N components producing N interleaved sets of images, wherein each set of images consists of different orders of the grating router and said each set is produced by one of the N components and
the output curve including N interleaved sets of reflective elements that have predefined phase shifts between the sets and are arranged so that all significant orders of each image set are reflected back through the router so as to efficiently transfer said particular input wavelength back to a selected one of the N waveguides, and wherein the selected waveguide is determined by preset phase shifts produced by the sets of reflective elements.
According to an operating method of my invention, for a waveguide grating router comprising a grating having an input curve and an output image curve, the input curve having N, N greater than 2, equally spaced waveguides connected thereto, the output curve having a plurality of spaced reflectors arranged thereon, the method includes the steps of
forming multiple paths through the router so as to transform a particular input wavelength applied to one of the N waveguides into N interleaved sets of equally spaced images on the output curve corresponding to different orders of the grating router;
reflecting back through the router, with predefined phase shifts from the N interleaved sets of reflective elements on the output curve, all significant orders of each image set so as to produce a single reflection of said particular input wavelength back to a selected one of the N waveguides, and
selecting one of the N waveguides using preset phase shifts produced by the sets of reflective elements.